Confocal UV and Resonance Raman Microscopic Imaging of Pharmaceutical Products

Abstract

Chemical imaging using confocal Raman microscopy is a useful analytical tool in drug development because of its ability to spatially image active ingredients and excipients in dosage forms and relate their distribution to product performance. While Raman spectra are highly specific for individual components of a formulation, most Raman microscopic mapping experiments require extensive experimental time. Laser wavelengths in the near-infrared range are used to suppress fluorescence but reduce sensitivity because of the inverse quadratic dependence of Raman scattering on laser wavelength. Compact, simple ultraviolet (UV) laser designs now allow for confocal UV Raman microscopy to be performed using a versatile instrument also capable of conventional Raman microscopy and epifluorescence imaging analyses. This study presents the results of UV Raman microscopy analyses using 266 nm laser irradiation of four pharmaceutical compositions of interest, including two types of tablets containing low doses of active ingredients (in the 0.2% w/w range), an amorphous dispersion containing 1% w/w of a small molecule drug, and an enteric coated layered peptide formulation. Resonance Raman enhancements are observed for four of the active ingredients studied in these formulations. The spectroscopic properties of the materials used in this study are also assessed by diffuse reflectance UV-visible spectroscopy, fluorescence spectroscopy, and conventional bulk Fourier transform Raman spectroscopy using 1064 nm laser irradiation. Confocal UV Raman microscopy was found to offer good sensitivity and allowed for rapid microscopic mapping of drugs and excipients at low concentrations in pharmaceutical formulations.

Recent Advances in Understanding the Micro- and Nanoscale Phenomena of Amorphous Solid Dispersions

“…For itraconazole/HPMC ASDs stained with both prodan and rhodamine-6G, Purohit et al observed that spin coated ASDs had smaller domain sizes than the film casted SD; domains as small as 300 nm in diameter were resolved . Confocal microscopy may also be paired with Raman spectroscopy, but the spatial resolution is still diffraction limited. − To achieve finer spatial resolution, ASD surfaces can be characterized using atomic force microscopy combined with infrared spectroscopy (AFM-IR). Through evaluation of the local infrared spectrum, the composition of a feature observed in the AFM topographical image may be qualitatively measured with sub-100 nm spatial resolution.…”

Nano-Raman Scattering Microscopy: Resolution and Enhancement

“…168,169 Resonant Raman imaging and analysis of pharmaceutical products were reported by Vogt and Strohmeier. 170 Since the wavelength of deep-UV light is much shorter than that of visible light, the spatial resolution with deep-UV excitation is higher than that with visible light. Figure 8C shows the effect of resolution enhancement.…”

“…Besides biological molecules, other organic and inorganic materials are electronically resonant in the deep-UV range. ,,,,,, Pharmaceutical materials are of particular interest with regard to deep-UV resonant Raman spectroscopy. , Resonant Raman imaging and analysis of pharmaceutical products were reported by Vogt and Strohmeier …”

Inhomogeneous Distribution of Components in Solid Protein Pharmaceuticals: Origins, Consequences, Analysis, and Resolutions

“…160,233,234 Inhomogeneity in small particles produced by spray-drying formulations may be hard to characterize due to the resolution limit of the confocal microscope, and focusing the laser beam on very small pixel can cause the sample burning. 235,236 Therefore, prior beam adjustment can be vital to image acquisition, including laser intensity and exposure time. In advanced microscopes equipped with interchangeable lasers and diffraction gratings, the excitation wavelength is also adjustable.…”